As is well known, an alternator is an electrical machine in which an alternating current is generated in its stator windings under the effect of inductor or rotor winding, through which an excitation current flows when the rotor winding is put into rotation. At the output of the stator windings, a diode bridge rectifies the alternating current so as to deliver a unidirectional, or direct current, voltage to the battery of the vehicle.
The rectified voltage has to be regulated in such a way as to remain constant regardless of the speed of rotation of the alternator, and regardless of the load on the battery. The output voltage of the alternator thus has a regulation waveform which reflects the excitation current, this waveform having a period T. The period T comprises a first phase T.sub.1 and a second phase T.sub.2 alternating with each other, i.e. having opposite signs whereby the amplitude increases in one phase and decreases in the other. In the first phase, the battery voltage is applied to the rotor winding, which enables the current flowing in the rotor winding to increase; while in the second phase, the battery voltage is not applied to the rotor winding, and this reduces the current in the rotor winding. The ratio T.sub.1 /(T.sub.1 +T.sub.2) is called the cyclic ratio.
It has previously been proposed to carry out the regulation of the rectified voltage output from the alternator using digital techniques. For example, European patent specification No. EP 0 481 862 describes a procedure for regulation by digital processing, in which the output voltage of the alternator is measured at each period T by sampling.
The numerical value of the measured temperature is then compared with a reference value; and from this comparison, the value of the alternating phases T.sub.1 and T.sub.2 in the next following period are deduced by modifying the cyclic ratio of a value directly proportional to the difference between the measured output voltage and the reference voltage.
A problem arises in the choice of the value that modifies the cyclic ratio. In this connection, if the duration of the excitation pulse in the phase T.sub.1 is short, and if this is followed by a severe increase in the load applied to the alternator, it is then desirable, in order to enable the system to react quickly, to increase very rapidly the duration of the successive excitation pulses T.sub.1. The purpose of this is to be able to make a large modification to the value of the cyclic ratio. This makes it necessary to multiply the difference between the measured voltage and the reference voltage by a high correcting factor.
On the other hand, in that case, the least variation between the measured voltage and the reference voltage will be repeated very severely on the excitation pulse. This leads to instability in the voltage regulation.